1. Field of the Invention
This invention relates to the carbothermic production of aluminum from aluminum oxide and a carbon-containing material. It particularly relates to apparatuses including a reduction furnace wherein alumina and the carbon are reacted by a carbothermic process to produce aluminum contaminated with a selected small amount of aluminum carbide and wherein losses of energy and of side-reaction products are minimized.
2. Description of the Prior Art
Reviewing the literature and the patent art readily indicates that there has been much activity by many people in an attempt to adequately define a thermal process which can compete advantageously with the conventional electrolytic methods of preparing aluminum. The art has long been aware of the many theoretical advantages which can flow from the use of a thermal reduction method for the production of aluminum as opposed to an electrolytic method. These advantages are becoming increasingly important as energy costs continue to increase. Unfortunately, the vast majority of such carbothermic processes have not resulted in a significant production of aluminum in a substantially pure state.
Specifically, these efforts have failed because they have invariably produced a mixture of aluminum metal and aluminum carbide. When such a mixture of 10-20% carbide or more cools to about 1400.degree. C., the aluminum carbide forms a cellular structure that entraps liquid aluminum; thus the mixture becomes difficult to pour. In consequence, unless extremely high temperatures are maintained throughout all of the steps, process manipulation of the mixture, in order to purify it, become extremely difficult, if not impossible.
The difficulty in producing aluminum with respect to thermal processes does not reside in the formation of the aluminum via reduction of the alumina-bearing ores, but rather, in the recovery of aluminum in a substantially pure state. The patent art, as well as the literature, is full of theories and explanations with respect to various back reactions which can take place between aluminum and the various carbon-containing compounds in the feed.
For example, U.S. Pat. No. 3,971,653 utilizes a slag containing an alumina mole fraction (N*=moles Al.sub.2 O.sub.3 /(moles Al.sub.2 O.sub.3 +moles Al.sub.4 C.sub.3) of 0.85 at a temperature of 2100.degree. C., with recycle of Al.sub.4 C.sub.3 -containing dross to the portion of the slag which is at reduction temperature. However, because the entire reaction to produce metal occurs at N*=0.85, the vaporization load is very high and the process power consumption is high.
U.S. Pat. No. 2,974,032 and U.S. Pat. No. 2,828,961 have described results that are typical of those to be expected from carbothermic reduction of a stoichiometric charge of alumina and carbon in a conventional electrically heated smelting furnace. The metal produced from the former process contains 20-37% Al.sub.4 C.sub.3 ; the metal produced by the latter process contains 20% Al.sub.4 C.sub.3. These processes are limited because reactive carbon and/or aluminum carbide is always present in contact with the metal that is produced and because time is available for the metal to react with the carbon and then to dissolve carbide up to its solubility limit.
One solution to the general problem of obtaining substantially pure aluminum from a carbothermic process is disclosed and claimed in U.S. Pat. No. 3,607,221. Although the process of this patent does result in the production of aluminum in a substantially pure state, extremely high operating temperatures are nevertheless involved which can lead to problems with respect to materials of construction. Another method for recovering substantially pure aluminum via a carbothermic process is disclosed and claimed in U.S. Pat. No. 3,929,456. The process of this patent also results in the production of substantially pure aluminum via a carbothermic process, but it does require careful control of the way the charge is heated in order to avoid aluminum carbide contamination.
By far, the most common technique disclosed in the prior art in attempting to produce aluminum of a high degree of purity has been directed to various methods of treating the furnace product which has conventionally contained about 20-35 weight percent of aluminum carbide. Thus, there are conventional techniques disclosed in the prior art, such as fluxing a furnace product with metal salts so as to diminish the amount of aluminum carbide contamination.
Unfortunately, the molten salts mix with the carbide so removed and it is costly to remove the carbide from the salts so that the carbide can be recycled to the furnace. Without such recycle, the power consumption and furnace size become uneconomical in comparison with prior methods practiced commercially for making aluminum.
U.S. Pat. No. 3,975,187 is directed towards a process for the treatment of carbothermically produced aluminum in order to reduce the aluminum carbide content thereof by treatment of the furnace product with a gas so as to prevent the formation of an aluminum-aluminum carbide matrix, whereby the aluminum carbide becomes readily separable from the alumina. Although this process is very effective in preserving the energy already invested in making the aluminum carbide, it requires a recycle operation with attendant energy losses associated with material handling.
In U.S. Pat. No. 4,099,959, a molten alumina slag is circulated through ducts, while being resistance heated in inverse relationship to the cross-sectional areas of the ducts, into alternating low and high temperature zones. The low-temperature zone is at a temperature high enough to produce aluminum carbide, and the high-temperature zone is at a temperature high enough to react aluminum carbide with alumina and produce aluminum. Off gases are first scrubbed through a first charge column containing only carbon and then through a second charge column containing only alumina in oder to preheat these charge materials without forming a "sticky" charge because of partial melting of aluminum oxycarbide. The low and high temperature zones operate entirely within the molten range for a slag composition within N* values of 0.82-0.85.
U.S. Pat. Nos. 3,929,456 and 4,033,757 disclose methods for carbothermically producing aluminum containing less than 20% Al.sub.4 C.sub.3, i.e., 5-10%, which comprise striking an open arc intermittently to a portion of the surface of the charge to be reduced.
However, advances have now been made in the art, wherein aluminum that is contaminated with about 20% aluminum carbide can be treated so as to obtain aluminum of commercial purity. One such technique is described in U.S. Pat. No. 4,216,010. This technique is adaptable to the production of aluminum containing less than 20% Al.sub.4 C.sub.3 (i.e., 10%). It comprises the step of contacting a product containing from 20-35% Al.sub.4 C.sub.3 with a melt rich in alumina in the absence of reactive carbon. Such purification techniques can impart commercial vitality to older carbothermic processes producing heavily contaminated aluminum. Thus it becomes worthwhile to locate the best existing prior art and to improve the effectiveness thereof.
In view of rapidly rising energy costs and regardless of the method that is employed to produce aluminum containing less than 20% Al.sub.4 C.sub.3, it is clear that measures must be taken to limit the energy lost to vaporized products, as one such improvement. Energy lost to vaporization depends on the amount of vapor produced in the reduction and decarbonization steps and also depends on the amount of vapor that is recovered in back reactions which release heat at times and places within the system where that heat released can be employed in pre-reduction reactions. There is also a need to minimize the quantities of product aluminum and of byproducts which escape from the hearth in order to minimize energy losses associated with these materials; to return vaporized materials to the reduction zone before undesirable reactions occur (such as Al.sub.2 O with oxygen in air), and maximize the proportion of Al.sub.4 C.sub.3 that is formed outside of the reduction zone.
The process U.S. Pat. No. 4,216,010 is effective with any amount of aluminum carbide contamination greater than about 2 weight percent. However, as indicated earlier, unless special procedures are used, e.g., U.S. Pat. Nos. 3,607,221 and 3,929,456, the amount of aluminum carbide contaminant which is produced by a so-called conventional reduction furnace ranges from about 20 to about 35 weight percent.
The process of U.S. Pat. No. 4,216,010 is directed particularly towards treatment of aluminum which is contaminated with from about 20 to about 35 weight percent of aluminum carbide, which is that amount of carbide contamination which is produced by a so-called conventional carbothermic reduction furnace, but it may also be used to treat aluminum which is contaminated with from about 2 to about 10 weight percent aluminum carbide as would be produced in furnaces used primarily for the production of aluminum such as those described in U.S. Pat. Nos. 3,607,221 and 3,929,456.
The novel process of U.S. Pat. No. 4,216,010 is carried out simply by heating the furnace product contaminated with aluminum carbide with a molten slag containing substantial proportions of alumina so as to cause the alumina in the slag to react with the aluminum carbide in the furnace product, thereby diminishing the furnace product in aluminum carbide. The expression "alumina in the slag to react with the aluminum carbide" is intended to describe various modes of reaction. While not wishing to be limited to a particular theory of operation, nevertheless, it appears that at least 2 modes of reaction as between the alumina in the slag and the aluminum carbide in the furnace product are possible.
One such mode can be described as the "reduction mode" and it involves reaction between the alumina in the slag and the aluminum carbide in the furnace product at reduction conditions so as to produce aluminum metal. One way of ascertaining operation in this mode is by the evolution of carbon monoxide.
Another such mode of reaction can be described as the "extraction mode" and it involves reaction between the alumina in the slag and the aluminum carbide in the furnace product so as to produce non-metallic slag compounds such as aluminum tetraoxycarbide, as opposed to producing liquid aluminum. Such "extraction mode" reactions occur at temperatures insufficient to cause reduction to produce additional aluminum and can occur without causing the evolution of carbon monoxide.
It is to be understood that said "extraction mode" can take place along with the "reduction mode".
In general, temperatures of at least 2050.degree. C. are necessary for the "reduction mode" operations at reaction zone pressures of one atmosphere. At any given pressure, the temperature required for "reduction mode" operation increases, as the level of aluminum carbide in the metal decreases. On the other hand, "extraction mode" operations can take place below 2050.degree. C.
Although a furnace with a roof forming a hearth shoulder to support the charge column thereabove provides satisfactory apparatus means for the control of charge to the hearth of the furnace, a method for controlling the amount of charge that is admitted to the hearth is generally more desirable. Such a method, moreoever, has the advantage that it can be useful in many furnaces of differing configurations, to control the amount of charge that is admitted to the hearth.